Hypothesis: Trymethylpsoralen shower PUVA is an effective therapy in the treatment of psoriasis. Specific aims: 1. To determine the dose-response relationship of TMP with addition of UVA in producing Mean Phototoxicity Dose (MPD) of aqueous shower delivery of TMP. 2. To ascertain the period of photosensitivity following TMP exposure. 3. To study the efficacy of TMP shower PUVA therapy in the treatment of psoriasis. TMP bath solution has been widely used in Europe but rarely in the United States. This is partially due to the cumbersome administration and disinfecting process required in between bathing each patient. In between treatments, hospital or clinic staff must drain, scrub and sterilize the tub prior to refilling (process takes 30 minutes). This makes this effective treatment not very time- or cost-effective in today's health care settings. We have completed a study of bath PUVA which demonstrated a 90% clinical effectiveness. it follows naturally that a shower can administer the drug in a more cost/time-efficient manner, since a cleaning solution can run through the same plumbing. We wish to determine if "bath" PUVA can be administered ina practical, cost-effective fashion through use of shower delivery.